A flat sheet can be formed to a single element curvature in either direction by simple bending. Adding a second direction of curvature requires stretching of the sheet. The contours of most wing skins on current model airplanes are a composite of chordwise and spanwise curvatures. These compound curvatures, called saddlebacks, are generally produced by shot peen forming the chordwise curvature and draping the spanwise curvature during the wing assembly operations. Only very large spanwise curves can be draped on assembly. Skins having severe spanwise curves require forming of both chordwise and spanwise curvature prior to assembly. Elevated temperature creep forming in large fixtures, pre-stress fixturing during shot peening, and press brake chip forming prior to shot peening are some of the methods used to form severe saddlebacks prior to assembly.
U.S. Pat. No. 3,668,912 teaches a centrifugal shot throwing wheel that may be used to obtain limited saddleback formation by shot peening on one side of a wing surface to obtain a curvature, placing shielding material adjacent a streamwise line, and shot peening one surface of the wing along the streamwise line.
U.S. Pat. No. 2,701,408 teaches a method of providing a curved surface by shot peening one side of a part as it passes through shot peening equipment. Should too great a curvature be obtained the part is turned over and passed through shot peening equipment using shot at a lower intensity.
It was found that a wing skin can be formed to a varying compound contour by selective shot peening.